A high electron mobility transistor (HEMT) using GaN-based materials can take full advantages of properties of the GaN-based materials owing to a two-dimensional electron gas (2DEG) layer which is naturally generated at a hetero interface between a GaN (gallium nitride) layer and an AlGaN (aluminum gallium nitride) layer. The HEMT has been attracting attention as a device to be used in a next-generation amplifier and switching device. One of performances required for the HEMT is a dielectric breakdown voltage. The dielectric breakdown voltage is a maximum voltage which can be applied between a source electrode and a drain electrode of the HEMT.
The GaN layer and the AlGaN layer of the HEMT are stacked on a semiconductor substrate via a buffer layer. However, the semiconductor substrate and the GaN layer are different from each other in lattice constant and thermal expansion coefficient. Therefore, the buffer layer, the GaN layer and the AlGaN layer include dislocations which are a kind of crystal defects. These dislocations cause a leakage current when a high voltage is applied to the HEMT. Furthermore, when the breakdown voltage of the HEMT is increased, carriers are generated between the semiconductor substrate and the buffer layer to form an inversion layer or an accumulation layer. As a result, a state like a short channel is generated between the source electrode and the drain electrode, thereby generating a leakage current. These leakage currents serve as barriers against improvements of the breakdown voltage of the HEMT.